commit ce1ff6f8a741079194c58d93027fff195cd12fcd
author Markus Handell <handellm@webrtc.org> Wed Jul 08 06:52:48 2020
committer Commit Bot <commit-bot@chromium.org> Wed Jul 08 07:59:03 2020
tree f0296cccf4167572ac7ee8e530ad50bdb28a626e
parent a827a30bb70c9d8592690ff59bb168106eae125f

Logging: reduce locking frequency by fixing TODO.

Bug: webrtc:11567
Change-Id: I0cd5062c3a088e3781d009242db32069193fbe82
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/176902
Commit-Queue: Markus Handell <handellm@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#31657}

rtc_base/logging.cc

diff --git a/rtc_base/logging.cc b/rtc_base/logging.cc
index bd2afcc..94ddcba 100644
--- a/rtc_base/logging.cc
+++ b/rtc_base/logging.cc
@@ -90,6 +90,7 @@
 // cleanup by setting to null, or let it leak (safe at program exit).
 ABSL_CONST_INIT LogSink* LogMessage::streams_ RTC_GUARDED_BY(g_log_mutex_) =
     nullptr;
+ABSL_CONST_INIT std::atomic<bool> LogMessage::streams_empty_ = {true};
 
 // Boolean options default to false (0)
 bool LogMessage::thread_, LogMessage::timestamp_;
@@ -269,6 +270,7 @@
   stream->min_severity_ = min_sev;
   stream->next_ = streams_;
   streams_ = stream;
+  streams_empty_.store(false, std::memory_order_relaxed);
   UpdateMinLogSeverity();
 }
 
@@ -281,6 +283,7 @@
       break;
     }
   }
+  streams_empty_.store(streams_ == nullptr, std::memory_order_relaxed);
   UpdateMinLogSeverity();
 }
 
@@ -438,12 +441,7 @@
 bool LogMessage::IsNoop(LoggingSeverity severity) {
   if (severity >= g_dbg_sev || severity >= g_min_sev)
     return false;
-
-  // TODO(tommi): We're grabbing this lock for every LogMessage instance that
-  // is going to be logged. This introduces unnecessary synchronization for
-  // a feature that's mostly used for testing.
-  webrtc::MutexLock lock(&g_log_mutex_);
-  return streams_ == nullptr;
+  return streams_empty_.load(std::memory_order_relaxed);
 }
 
 void LogMessage::FinishPrintStream() {

rtc_base/logging.h

diff --git a/rtc_base/logging.h b/rtc_base/logging.h
index 0aa1e67..1e5f725 100644
--- a/rtc_base/logging.h
+++ b/rtc_base/logging.h
@@ -46,6 +46,7 @@
 
 #include <errno.h>
 
+#include <atomic>
 #include <sstream>  // no-presubmit-check TODO(webrtc:8982)
 #include <string>
 #include <utility>
@@ -557,6 +558,12 @@
   // The output streams and their associated severities
   static LogSink* streams_;
 
+  // Holds true with high probability if |streams_| is empty, false with high
+  // probability otherwise. Operated on with std::memory_order_relaxed because
+  // it's ok to loose or log some additional statements near the instant streams
+  // are added/removed.
+  static std::atomic<bool> streams_empty_;
+
   // Flags for formatting options
   static bool thread_, timestamp_;